In 2267, when Spock began suffering from pon farr, he returned to Vulcan to mate with T'Pring.
T'Pring, however, preferred Stonn instead, and executed her right to claim kal-if-fee at the koon-ut-kal-if-fee ceremony (a Vulcan wedding). She picked James Kirk as her champion, and he accepted while being unaware that the fight was to be to the death.
T'Pring reasoned that if Spock won, he would reject her for having chosen another, and she would have Stonn. If Kirk won and killed Spock, he would not want her, and she would still have Stonn. However, should Spock still accept her, he would likely be gone for several extended periods of time following his career in Starfleet, thus allowing her to be with Stonn.